Current technology trends have been able to package consumer music playback devices into extremely convenient sizes, enabling more frequent casual listening opportunities than ever before. However, prior art consumer music playback devices suffer from a number of shortcomings. For example, a problem with prior art devices is their inability to deliver a consistent perceived loudness level from song to song, often because the dynamic range of professionally distributed music varies greatly. Environmental noises also contribute to changes in the perceived loudness level. Specifically, a loud environmental noise can overpower quiet sounds. Such changes in perceived loudness level between songs cause a frustrated user to repeatedly manually modify the volume control of such prior art devices, preventing a completely passive listening experience.
A problem with providing a consistent loudness between audio tracks is twofold: first, a ratio between signal peaks and average signal level can vary greatly, and second, human perception of loudness is not directly correlated with signal peaks or average signal level. Some prior art portable consumer music playback devices attempt to calculate a consistent loudness over audio tracks by lowering the overall track output, e.g. normalization, of louder tracks to match quiet tracks. For example, Apple Computer Inc. of California has created a complex algorithm, Sound Check™, to accurately detect a signal power of an audio track over a variety of frequencies to adjust for human perception of loudness and other criteria. However, actual results can be perceived as poor. Sound Check™ does not adequately produce consistent loudness levels in the processed audio tracks. Furthermore, as illustrated in FIG. 1, the reduced overall signal level by normalization does not make use of an entire available dynamic range of a consumer music playback device, lowering signal to noise ratio and thereby degrading overall sound quality. The complex algorithm is currently used in Apple iPod™ products.
Other prior art music playback devices use a dynamic range compressor to modify the signal peaks and long-term loudness levels of an audio track to maximize levels while providing a consistent loudness. The dynamic range compressor reduces a gain of an audio signal if its amplitude exceeds a threshold. Unlike normalization, dynamic compression applies varying levels of gain over the audio signal. However, this dynamic compression technique alone does not work well when signal transients are very fast, requiring attack times, and sometimes release times, of the dynamic range compressor to be also very fast, thus producing a great deal of distortion in a processed signal. When the dynamic range compressor is slower than the signal transients, the signal peaks overshoot the available dynamic range of the consumer music playback device, causing audio clipping.
The concept of look-ahead limiter is not new. For example, radio stations use look-ahead limiters. A look-ahead limiter anticipates and controls peaks by introducing a delay thereby allowing the device to effectively look ahead in time and anticipate changes necessary to the audio signal to fit within the allowable range. In order for the look-ahead limiter to maximize lower signal levels, extremely fast attack and release times are used to manipulate signal peaks. Specifically, large signal peaks are reduced as quickly as possible. However, similar to a dynamic range compressor, fast attack and release times create audible distortion in prior art and look-ahead limiter systems.
Music distribution channels, like radio stations, use look-ahead limiters to significantly reduce the signal peaks and achieve a more consistent average signal power. Specifically, a look-ahead limiter measures the signal peaks and applies an automatic gain control to a delayed version of a raw signal to reduce the magnitude of signal peaks. Once the signal peaks are controlled, music levels are maximized for radio broadcast. The look-ahead limiter is typically complex and sophisticated in nature. As such, the look-ahead limiter is too difficult for a consumer to use properly. Furthermore, the look-ahead limiter is computationally too expensive for the limited processing capabilities of portable consumer music playback devices. This makes the look-ahead limiter economically impractical, especially in personal portable players.
The present invention addresses at least these limitations in the prior art.